Final Words

It's always great to see manufacturers improving their existing products but the 1.02 firmware for the M5 Pro doesn't really change its ranking. It provides minor tweaks to random IO performance but when looking at the big picture, the changes are fairly insignificant. The M5 Pro is still noticeably behind Samsung's 840 Pro and OCZ's Vector, which are currently in their own class when it comes to performance. Plextor's pricing is, however, pretty competitive and depending on the capacity you can get the M5 Pro for as much as $100 cheaper than the 840 Pro or Vector. Plextor has turned out to be one of the more reliable SSD vendors, although admittedly their customer base isn't as large as some of the other players we cover.

Despite Plextor's reliability, I'm not very comfortable with the high IO latency in the M5 Pro. I would rather have slower peak performance if it translated to more consistent overall performance as the end user is not going to notice the peaks but is definitely going to notice the hiccups caused by frequent, high maximum latencies.

Our Storage Suites are run on an empty drive, whereas in the IO consistency test the drive is first filled with sequential data before being hammered by 4KB random writes. The Storage Suites also consist of various IOs with different transfer sizes, queue depths and data patterns and as we have shown before, sequential writes recover performance with most SSDs. The SSD is also not being subjected to IO load all of the time, there are lots of idle periods where the SSD can do GC to recover performance.

So, our Storage Suites don't fully ignore IO consistency but it's hard to say how much of an impact the M5 Pro's IO consistency has on its scores. Reply

You need to examine the latency for this SSD to see what it is doing. Like you, I was surprised when I first saw the M5P dropping down to such low IOPS under sustained heavy load. Basically, the M5P is rapidly switching between two modes -- a slow throughput mode (presumably doing GC) and a high throughput mode. It certainly does not look pretty when you plot it out.

But there are two (possibly) mitigating factors:

1) The average throughput isn't terrible, especially with at least 20% OP. The more OP, the greater percentage of time the SSD spends in the high throughput mode, thus raising the average throughput. The average throughput still is not as good as the Vector, Neutron, or 840 Pro, but it is not as bad as it looks on the graph.

2) Importantly, Plextor appears to put an ABSOLUTE cap on worst-case latency. I have never seen the latency go over 500ms, no matter what you throw at it. For comparison, with the Samsung 840 Pro, with no OP and a very heavy load, the latency will, very occasionally, go over even 1000ms. You can easily see the bimodal distribution of latencies for the Plextor if you look at the normal probability scale CDF plot. It seems that Plextor has tuned the firmware so that whenever it is in the slow mode doing GC, it has an absolute limit of 500ms before any IO returns. I guess the price to be paid for that absolute latency cap is that the average and worst-case throughput is lower than the competition -- but not so much lower that nobody could consider it an acceptable trade-off in order to gain the absolute cap on worst-case latency.

Personally, I would still choose the 840 Pro over the Plextor for sustained heavy workloads (I would overprovision either SSD by at least 20%) because the 840 Pro has much better average latency. But I can imagine that some applications might benefit from the absolute 500ms cap on worst-case latency that the Plextor provides.

Note that none of this really matters for the consumer workloads most people would put an SSD under. Under most consumer workloads, neither the Plextor nor any of the others would have performance drops anywhere near as bad as shown in these sustained heavy workload conditions.Reply

The max latency constraint is very clear from the graphs you generated. It's not that the firmware is "bad" necessarily, it is just optimizing for a different performance measurement, one that Anandtech doesn't cover.

I think an analysis of whether max latency is ever important than max throughput would be interesting, along with some data on how the Plextor compares to other drives in this alternate metric.Reply